1. Field of the Invention
The present invention relates to a wireless power transmission system, which includes a power transmitter and a power receiver and which transmits a data signal from the power receiver to the power transmitter, and vice versa, while transmitting power from the power transmitter to the power receiver.
2. Description of the Related Art
Recently, as cellphones, terrestrial digital TV broadcasting and their related technologies have been further advanced, wireless receivers that receive text data, audio data and telecasts wirelessly as radio waves without using a cable connection have become more and more popular. Currently, electric power is still supplied through a cable to most of those wireless receivers in order to charge its built-in battery to the point that the device is ready to use. However, as those wireless telecommunications technologies have been developing, a lot of people have been attempting nowadays to transmit electric power, as well as those data signals, wirelessly, too.
For example, wireless power transmission technologies by electromagnetic induction have already been applied to various consumer electronic products such as electric shavers and motorized tooth brushes, and have increased their handiness for general consumers successfully. For instance, Japanese Patent Application Laid-Open Publication No. 2004-206245 (which will be referred to herein as Patent Document No. 1 for convenience sake) discloses a configuration for a system of which the power transmitter and power receiver have their coils coupled together electromagnetically to transmit power wirelessly from the power transmitter to the power receiver and which transmits data from the power transmitter to the power receiver, and vice versa.
Hereinafter, it will be described with reference to FIG. 10 how the system of Patent Document No. 1 works.
FIG. 10 illustrates a configuration for a conventional non-contact data reading and writing system. In this system, a reader/writer (power transmitter) 911 and a non-contact IC card (power receiver) 992 are electromagnetically coupled together to read and write data by a non-contact method. The wireless transmitting section 131 of the reader/writer 911 generates power and a data signal to be transmitted to the non-contact IC card 992. For that purpose, a periodic signal generator 61 outputs a periodic signal, of which one period is represented by substantially the same frequency fo as the resonant frequency of a transmitting-end resonant circuit 91 and a receiving-end resonant circuit 92. Based on a data signal 111 to be transmitted to the non-contact IC card 992, a modulator 9 outputs a waveform that has been modulated with the data signal 111 by using the periodic signal supplied from the periodic signal generator 61 as a carrier. Next, an amplifier amplifies the output of the modulator 9 to required amplitude. Then, a matching circuit 21 achieves impedance matching and outputs a waveform including power and the data signal in combination that has been obtained by modulating the periodic signal with the data signal 111. This is the output of the wireless transmitting section 131.
The output of the wireless transmitting section 131 is supplied to a circulator 171, which passes an input from the matching circuit 21 to the transmitting-end resonant circuit 91, an input from the transmitting-end resonant circuit 91 to a wireless receiving section 141 and an input from the wireless receiving section 141 to the matching circuit 21, respectively. It should be noted that actually, the wireless receiving section 141 rarely outputs anything to the matching circuit 21. Thus, it can be said that the output of the wireless transmitting section 131 is almost always passed by the circulator 171 to the transmitting-end resonant circuit 91.
Since the transmitting-end and receiving-end resonant circuits 91 and 92 are electromagnetically coupled together by a non-contact method via electromagnetic induction, the output of the wireless transmitting section 131 is transmitted to an IC chip 32 by way of the transmitting-end resonant circuit 91 and the receiving-end resonant circuit 92. In response, the IC chip 32 gets power to drive itself from the data signal transmitted and receives the data signal. In this manner, power and data are transmitted from the reader/writer 911 to the non-contact IC card 992.
Next, it will be described how to transmit data from the non-contact IC card 992 to the reader/writer 911. The IC chip 32 changes the value of the load resistance (not shown) of the receiving-end resonant circuit 92 according to the data signal to be transmitted. If the value of the load resistance is changed, the impedance of the transmitting-end resonant circuit 91 as viewed from the circulator 171 changes in the reader/writer 911. As a result, among various signals transmitted inside the reader/writer 911, a signal that has been output from the circulator 171 to the transmitting-end resonant circuit 91 and then reflected back from the transmitting-end resonant circuit 91 to the circulator 171 has its waveform affected by the value of the load resistance of the receiving-end resonant circuit 92 of the non-contact IC card 992.
The circulator 171 outputs that signal waveform that has been reflected back to itself to the wireless receiving section 141. In response, a demodulator 101 demodulates the data signal by sensing the variation in the signal waveform due to the change of the load resistance, and outputs a received signal 121 as the data signal that has been transmitted from the non-contact IC card 992 to the reader/writer 911.
By using the circulator in this manner, the output of the wireless transmitting section 131 is transmitted to the non-contact IC card 992 just as intended without being diverted to the wireless receiving section 141. As a result, the power loss can be cut down. In addition, the reflected wave of the transmitting-end resonant circuit 91 that has been produced due to the change of the load resistance is input to only the wireless receiving section 141, not to the wireless transmitting section 131. Consequently, it is possible to prevent a variation in the signal waveform from decreasing to a hardly sensible level, and therefore, the demodulator 101 can demodulate the data signal easily.
Using such a configuration, power is transmitted from the reader/writer (power transmitter) 911 to the non-contact IC card (power receiver) 992 and data can be transmitted from the power receiver to the power transmitter, and vice versa.
However, whenever a circulator is included in a power transmitter, insertion loss due to the insertion of the circulator is inevitable.
Such an insertion loss is non-negligible in those applications that require medium to high powers to drive AV devices and electric vehicles, which have been developed more and more extensively these days. This is because in those medium to high power applications, the power transfer efficiency achieved should be as close to what is achieved by cable connection as possible, and therefore, factors that would cause some loss should be eliminated as much as possible.
It should be noted that the technology disclosed in Patent Document No. 1 is supposed to be applied to transmitting power and data to a non-contact IC card and the power to be transmitted is on the order of just a few μW. That is why in the applications intended by Patent Document No. 1, such insertion loss is not a problem.
Also, the system shown in FIG. 10 is used mainly to transmit and receive data and the wireless transmission of power is required just for the purpose of transmitting and receiving data. Conversely, if the main purpose is to transmit power wirelessly rather than to transmit and receive data, then the power transfer efficiency should be high.
In order to achieve high power transfer efficiency, a class E amplifier is often used. A class E amplifier is known as a circuit that can convert a DC voltage supplied from a DC power supply into an AC signal efficiently. Unlike a class A amplifier or a class B amplifier, a class E amplifier does not amplify an input signal but receives an input periodic signal as a trigger and converts a DC voltage into an AC voltage efficiently. A class E amplifier can be used effectively in a system that generates a single frequency and that transmits power from a power transmitter to a power receiver using a signal with a particular resonant frequency.
With such efficient wireless power transmission ensured, introduction of the ability to transmit and receive data should also be taken into consideration. For example, to transmit power wirelessly and safely so that the power being transmitted is not stolen by a third party, authentication and arbitration should be done between a power transmitter and a power receiver. For that purpose, a bidirectional data transmission capability, including the ability to transmit data from a power receiver to a power transmitter and the ability to transmit data from the power transmitter to the power receiver, is required to say the least.
It is therefore an object of the present invention to provide a wireless power transmission system that can achieve high power transfer efficiency without using a circulator or any other device to cause some insertion loss and that can transmit data bidirectionally from a power receiver to a power transmitter, and vice versa.